The present invention relates to a mold for pressing recorded discs and particularly for pressing thermoplastic high density information recorded discs, such as video discs.
The molds which have been used for pressing recorded discs generally include a base plate and a cap plate mounted on and secured to the base plate. The cap plate has a mold cavity in its outer surface and channels in its inner surface which oppose the surface of the base plate. The channels are connected through passages in the base plate to inlet and outlet ports whereby cooling and heating media can be passed through the channels to control the temperature of the surface of the mold cavity. A heating media, such as steam, is used during the molding cycle to heat the thermoplastic molding material so that it can flow to fill the mold cavity. A cooling media, such as water, is used to harden the molding material after the disc is formed. A stamper is mounted on the outer surface of the cap plate across the mold cavity. The stamper is a thin metal plate having on its surface the negative of the surface relief pattern to be formed on the surface of the recorded disc. The outer edge of the stamper is secured to the plate by a ring which holds the stamper tightly against the surface of the cap plate. The recorded disc is pressed by two molds having opposed mold cavities which together form the shape and size of the desired disc.
A problem in pressing molded recorded discs is to eliminate surface defects, such as voids and air blisters. This is particularly desirable for high density information discs where even small surface defects can extend across and adversely affect a relatively large amount of the information on the disc. One factor which can affect surface defects is the proper filling of the cavity of the mold by the molding material. The proper filling of the mold cavity can be affected by the shape of the mold cavity and the control of the heating and cooling of the cavity. Surface defects can also be caused by mechanical factors, such as poor securing of the stamper to the cap plate and mechanical defects in the surface of the cavity in the cap plate.